Optically variable devices are used in a wide variety of applications, both decorative and utilitarian; for example, such devices are used as security devices on commercial products. Optically variable devices can be made in numerous ways to achieve a variety of effects. Examples of optically variable devices include the holograms imprinted on credit cards and authentic software documentation, color-shifting images printed on banknotes and enhancing the surface appearance of items such as motorcycle helmets and wheel covers.
Optically variable devices can be made as a film or foil that is pressed, stamped, glued, or otherwise attached to an object, and can also be made using optically variable pigments. One type of optically variable pigments is commonly called color-shifting pigments because the apparent color of images appropriately printed with such pigments changes with the change of the angle of view and/or illumination. A common example is the numeral “20” printed with color-shifting pigments in the lower right-hand corner of a U.S. twenty-dollar bill, which serves as an anti-counterfeiting device.
Optically variable devices can also be made with magnetic pigments that are aligned with a magnetic field. After coating a product with a liquid composition, a magnet with a magnetic field having a desirable configuration is placed on the underside of the substrate. Magnetically alignable flakes dispersed in a liquid organic medium orient themselves parallel to the magnetic field lines, tilting from the original orientation. This tilt varies from normal to the surface of a substrate to the original orientation, which included flakes essentially parallel to the surface of the product. The planar oriented flakes reflect incident light back to the viewer, while the reoriented flakes do not.
Some anti-counterfeiting devices are covert, while others are intended to be noticed. Unfortunately, some optically variable devices that are intended to be noticed are not widely known because the optically variable aspect of the device is not sufficiently dramatic. For example, the color shift of an image printed with color-shifting pigments might not be noticed under uniform fluorescent ceiling lights, but is more noticeable in direct sunlight or under single-point illumination. This can make it easier for a counterfeiter to pass counterfeit notes without the optically variable feature because the recipient might not be aware of the optically variable feature, or because the counterfeit note might look substantially similar to the authentic note under certain conditions.
In order to make optical security devices highly noticeable, relatively high concentrations of magnetic flakes are used for forming bright images which e.g. appear to be three-dimensional images. The resulting optical devices do not transmit light. Sometimes they are referred to as security patches, and there is a security risk associated with possible transfer of a patch to a forged document. Additionally, the opaque security devices formed of magnetic pigments may be undesirable because they essentially hide the background pattern whereas intrinsic Guilloché patterns are widely relied upon in banknote printing.
Accordingly, there is a need to mitigate the disadvantages of existing security patches. It is an object of the present invention to provide highly noticeable security devices where optical effects produced by magnetically aligned pigments cooperate with the underlying printed patterns, and which may be formed within high speed printing processes.